The present invention pertains generally to testing of printed circuit assemblies, and more particularly to a low-cost wireless test fixture adapter for a printed circuit assembly tester.
Printed circuit assemblies (PCA""s) must be tested after manufacture. PCA testing may be categorized into bare-board testing and loaded-board testing. During bare-board testing, a bare printed circuit board without components and devices attached is tested to verify the continuity of the traces between pads and/or vias on the board. During loaded-board testing, a printed circuit board with some or all of the electrical components and devices attached is tested in order to verify that all required electrical connections have been properly completed. Loaded-board testing may also include integrated circuit (IC) testing, which is performed to verify that the loaded components perform within specification.
In previous years, PCA""s were designed and manufactured so that their electrical contacts which were probed during test were arranged in a regularly spaced pattern. During testing, the PCA was placed directly atop a regularly spaced pattern of interface probes located in the tester. As PCA and component geometries shrunk, PCA contact pads could no longer be placed in a regularly spaced pattern and probed directly by interface probes. A bare-board fixture was developed which utilized long, leaning solid probes to provide electrical connections between small, closely spaced, randomly located targets on the PCB and regularly spaced interface probes located in the tester.
A bare-board tester probes testpads, vias, and plated through-holes on bare printed circuit boards only and tests for electrical connectivity and continuity between various test points in the circuits on the printed circuit boards before any components are mounted on the board. A typical bare-board tester contains test electronics with a large number of switches that connect test probes to corresponding test circuits in the electronic test analyzer.
Although each bare-board fixture builder uses unique components and manufacturing processes, most bare-board fixtures resemble that shown in FIG. 1 and include regularly spaced spring probes 114 on a testbed 112 of a tester and long, solid test probes 102 and 116 inserted through several layers of guide plates 100 drilled with small through-holes and held in a spaced-apart fashion with spacers 110. The bed of standard spring probes 114 actuate the solid test probes 102 and 116. The long, solid probes may be inserted into the guide plates vertically or at an angle in order to facilitate an easy transition between the fine-pitch, or very close, spacing of testpads 104 and 106 on the PCB side of the fixture and the larger-pitch spacing of the spring probes on the tester side of the fixture. One such bare-board fixture is disclosed in U.S. Pat. No. 5,493,230 titled xe2x80x9cRetention of Test Probes in Translator Fixturesxe2x80x9d to Swart et al., which is incorporated herein for all that it teaches.
Existing bare-board fixtures can consistently hit test targets equal to or greater than 20 mils in diameter with equal to or greater than 20-mil pitch (center-to-center spacing). Unfortunately, heretofore, it is not been possible to use bare-board fixtures directly on a loaded-board tester because there are many unique features which render bare-board test equipment directly incompatible with loaded-board test equipment.
Prior art bare-board fixtures are not designed to accommodate printed circuit boards (PCBs) which are populated with electronic components; only PCB features which are flush with respect to the PCB (pads, vias, and plated through holes) can be probed. Bare-board testers are used to determine the connectivity and continuity of test points and circuitry in a PCB. Unlike bare-board testers, loaded-board testers cannot tolerate higher electrical resistance between a target on a PCB and the tester electronics. Loaded-board fixtures must provide low-resistance connections and interfaces between targets, fixture components, and tester electronics. Unlike loaded-board testers, bare-board testers cannot determine whether a component or a group of components exists and functions properly.
The probe spacing of bare-board fixtures which are designed to fit on bare-board testers is not generally compatible with the interface probe spacing of loaded-board testers. Bare-board fixtures translate a target on the PCA under test to the nearest interface probe in the bare-board tester. However, loaded-board tester resources must be uniquely assigned and linked to specific targets and circuits. In loaded-board testing, the nearest interface probe may not be appropriate for a given target. Bare-board fixtures are not able to provide unique electrical routing to adjacent, nonadjacent, and remote tester resources, cannot reach remote resources, and cannot provide the complex, loaded-board resource routing patterns required by a loaded printed circuit board.
FIG. 2 illustrates a first and second embodiment of a prior art loaded-board, guided-probe test fixture. The test fixture of the first embodiment comprises two major assemblies. The first assembly 240 is a translator fixture comprising a series of vertically spaced-apart and parallel guide plates 216, which are supported in parallel by solid posts 222 that hold the fixture together as a solid unit. The fixture also includes an array of leaning probes 226 extending through guide holes in the translator guide plates 216. The leaning probes 226 are in alignment on a first side of the translator fixture 240 with test targets 220 of a loaded circuit board 218. The leaning probes 226 are in alignment on a second side of the translator fixture 240 with spring probes 214 on a first side of a probe-mounting plate 224. The long leaning probes 226 are used to facilitate an easy transition from the fine-pitch targets 220 on the device under test 218 and larger pitch targets (spring probes 214) on the probe-mounting plate 224.
Probe-mounting plates are well known in the art; one such plate being a probe-mounting plate made of glass-reinforced epoxy. Personality pins 228 are embedded on a second side of the probe-mounting plate 224 and the personality pins are electrically connected to the spring probes 214 by wires 230. The wirewrap posts 232 of the personality pins 228 pass through holes in an alignment plate 234 to make contact with tester-to-fixture interface probes 200 which interface the fixture to the pins of the testhead (not shown). Tester-to-fixture interface probes 200 are in a predetermined fixed, regularly spaced pattern and are arranged to electrically contact the test pins of the testhead at one end. The alignment plate 234 aligns the wirewrap posts 232 of personality pins 228 to correspond to the predetermined location of the tester-to-fixture interface probes 200. The second major assembly 242 of the first embodiment is the unit of the probe-mounting plate 224 containing spring probes 214 and personality pins 228 and the alignment plate 234 which aligns the wirewrap posts 232 of the personality pins 228 with the tester-to-fixture interface probes 200.
Accurate alignment of the test fixture is essential for reliable operation. Alignment for the printed circuit board 218 to the translator fixture 240 is maintained by means of tooling pins (not shown), which is well known in the art of board test. Alignment between the translator fixture 240 and the probe-mounting plate 224 is maintained by means of alignment pins (not shown) or other known means. Alignment between the alignment plate 234 and the tester-to-fixture interface probes 200 is controlled through the mounting and locking hardware well known in the art of loaded-board test.
The method of operation of the test fixture is as follows. The translator assembly 240 is mounted on the probe-mounting plate/alignment plate assembly 242. The entire fixture, which includes the translator fixture 240 and the probe-mounting plate/alignment plate assembly 242 is then mounted on the regularly spaced tester-to-fixture interface probes 200. Next the loaded printed circuit board 218 to be tested is placed on the translator fixture assembly 240 by means of tooling pins (not shown). The test targets 220 of the loaded-printed circuit board 218 are then brought into contact with the leaning probes 226 of the translator fixture assembly 240 by any of several known means, including vacuum, pneumatic or mechanical actuating means. As the printed circuit board 218 is drawn toward the tester (not shown), the leaning probes 226 are sandwiched between the test targets 220 of the printed circuit board 218 and the spring probes 214, thus making a good, low-resistance contact between the tips of leaning probes 226 and test targets 220. The spring force of the spring probes 214 helps the tips of leaning probes 226 make a good contact with the test sites 220. Once electrical contact between the loaded printed circuit board and the leaning probes 226 is established, in-circuit or functional testing may commence.
As illustrated, the unit 242 which includes the probe-mounting plate 224, spring probes 214, personality pins 228, alignment plate 234, and wirewrap posts 232 is complicated and time consuming to build. Advances in fixture technology has led to what are known as xe2x80x9cwirelessxe2x80x9d fixtures. The test fixture of the second embodiment shown in FIG. 2 illustrates the features of a wireless fixture. As illustrated, the wireless fixture comprises two major assemblies. The first assembly 246 is a translator fixture comprising a series of vertically spaced-apart and parallel guide plates 216, which are supported in parallel by solid posts 222 that hold the fixture together as a solid unit. The fixture 246 also includes an array of translator pins such as leaning probes 226 extending through guide holes in the translator plates 216. The leaning probes 226 are in alignment on a first side of the translator fixture with test targets 220 on printed circuit board 218. The leaning probes 226 are in alignment on a second side of the translator fixture 246 with double-headed spring probes 208 on a first side of a probe-mounting plate 206.
Double-headed spring probes 208 extend through a second side of the probe-mounting plate 206 and make electrical contact with contact pads 212 on a wireless interface printed circuit board (WIPCB) 202. The contact pads 212 on the first side of the PCB 202 are electrically connected to contact targets 204 on a second side of the PCB 202. Contact targets 204 on the second side of the wireless interface PCB 202 are patterned to correspond with tester-to-fixture interface probes 200 which interface the test pins of the testhead to the fixture. Tester-to-fixture interface probes 200 of the tester are in a predetermined fixed, regularly spaced pattern. The wireless interface PCB 202 allows the double-sided spring probes 208 to correspond to predetermined locations of the tester-to-fixture interface probes 200 by means of copper traces from the contact pads 212 that correspond to the locations of the double-headed spring probes 208 to contact pads 204 that correspond to the locations of the tester-to-fixture interface probes 200 of the tester. The second major assembly 248 of the second embodiment is the unit of the probe-mounting plate 206 containing double-sided spring probes 208 and the wireless interface PCB 202 which aligns the double-sided spring probes 208 with the tester-to-fixture interface probes 200.
Alignment for the printed circuit board 218 to the translator fixture 246 is maintained by means of tooling pins (not shown), which is well known in the art of board test. Alignment between the translator fixture 246 and the probe-mounting plate 206 is maintained by means of alignment pins (not shown) or other known means. Alignment between the probe-mounting plate 206 and the wireless interface PCB 202 is maintained by means or alignment pins (not shown) or by other known means. Alignment between the wireless, interface PCB 202 and the tester-to-fixture interface probes 200 is controlled through mounting and locking hardware well known in the art of loaded-board test.
The method of operation of the test fixture is as follows. The translator assembly 246 is mounted on the probe-mounting plate/interface PCB assembly 248. The entire fixture, which includes the translator assembly 246 and the probe-mounting plate/interface PCB assembly 248 is then mounted on the regularly spaced tester-to-fixture interface probes 200 on the tester. Next the loaded printed circuit board 218 to be tested is placed on the translator fixture assembly 246 by means of tooling pins (not shown). The test targets 220 of the loaded-printed circuit board 218 are then brought toward the tester by any of several known means, including vacuum, pneumatic or mechanical actuating means. As the printed circuit board 218 is drawn toward the tester, the leaning probes 226 are sandwiched between the test targets 220 of the printed circuit board 218 and the double-headed spring probes 208, thus making a good, low-resistance contact between the tips of leaning probes 226 and test targets 220. The spring force of the double-headed spring probes 208 helps the tips of leaning probes 226 make a good contact with the test sites 220.
As component and board geometries shrink and become denser, loaded-board testing becomes more difficult using standard fixtures, resulting in more reliance on wireless fixture interfaces. Current wireless test fixtures include a tester-to-fixture interface which interfaces between the testhead probes and the fixture, a PCA interface which interfaces between the board under test and the fixture, and a fixture interface printed circuit board interfacing the tester interface and board interface. The tester interface is typically universal in that it does not change from one design of a board under test to the next. In contrast, the board interface and fixture interface printed circuit board are customized for the specific design of the board under test.
In current designs, the fixture interface PCB is positioned below the probe plate that bears the load of the probes. This design is advantageous because the fixture interface PCB is protected from damage since it lies within the fixture. However, the design is also problematic due to the difficulty in accessing the fixture interface PCB. Furthermore, current fixture designs include each of the tester-to-fixture interface, fixture interface PCB, and fixture-to-PCA interface. Thus, within a single fixture, the tester-to-fixture interface, fixture interface PCB, and fixture-to-PCA interface are all intertwined. In other words, the universal and customized portions of the fixture are each interdependent on one another. Thus, each time a fixture is manufactured for a particular board design, the universal tester-to-fixture interface must also be replicated. This adds significant weight and cost to the fixture.
Accordingly, a need exists for a fixture design which separates the universal and customized portions from one another to allow the universal portions of the fixture, such as the tester-to-fixture interface, to be reused for each board design. A need also exists for better access to the fixture interface PCB. It would also be desirable to reduce the weight and cost of the fixture. It would also be desirable to use the same tester-to-fixture interface for both bare-board testing and loaded-board testing.
The present invention is a test fixture adapter which implements the universal portions of the fixture interface and allows mounting of a customized fixture interface printed circuit board thereon. According to the invention, the fixture interface PCB and the fixture-to-PCA-under-test interface are designed to be completely separate. The testhead-to-fixture interface lies below the customized parts and is intended to be universal. The adapter may be reused from one board design to the next, requiring only that the fixture interface printed circuit board and customized portions of the fixture be redesigned and manufactured. In particular, only a single fixture adapter containing a universal testhead-to-fixture interface need be designed and employed for use with all fixtures. Because only the customized portions of the fixture are implemented in the fixture, this results in reduced cost and weight to the fixture. In addition, because the fixture interface PCB is separately mounted on the adapter, simpler access to the fixture interface PCB is facilitated.
An additional advantage of the invention is that the universal testhead-to-fixture interface may be compatible with both bare-board and loaded-board testing. In particular, the invention allows simple coupling of probes that do not require sockets such as bare board fixtures.